On board to take journey - it's happening (2,5)
I believe the answer is:
in train
'on board' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.
'take journey it's happening' is the wordplay.
I cannot really see how this works, but
'take' could be 'train' (synonyms) and 'train' is located in the answer.
'happening' could be 'in' (happening can mean fashionable or 'in') and 'in' is present in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
'to' acts as a link.
Can you help me to learn more?
(Other definitions for in train that I've seen before include "Being on board" , "... properly directed" , "making progress" , "Moving forward" , "Under way" .)